cover over a medium voltage duct bank

[mshields]

What do you think?

I've got a 5kV duct bank running under a road. Excavation of that road has revealed a sewer line at a depth of 38 inches (to the top). If we were to put our 16 inch tall ductbank directly on top of it, we'd be left with 22 inches.

a) what do we need for a buffer between the two utilities and can we/should we use some kind of expansive material?
b) baring going deep (which is feasible), the contractor has suggested and even had blessed by the local AHJ, pouring 6 inches of concrete around the duct bank in lieu of 3. So another 3 inches on the bottom, another 3 inches on the top leaving the top of the duct bank (assuming 3 inch buffer between utilities) about 11 inches.

I'd be interested in your collective experience on the matter and whether or not you think I should accept it?

 

[kingpb]

Going on top with the duct bank will probably crush the sewer line unless additional support is provided to protect it.

Pouring more concrete around it will change the thermal properties and possibly cause an increase in cable size.

Going deeper could also change thermal properties and cause an increase in cable size.

My recommendation is to get a civil engineer involved who is experienced in duct banks. They will be better at assessing your options.

 

[jdsmith]

As kingpb pointed out the thermal design of the duct bank needs to be considered with any changes in the concrete. I am aware of situations very similar to yours where 30-36" of cover was desired and obstructions were encountered in the 36-48" depth range. In those cases the civil folks suggested making sure the fill around the sewer line was compacted well, then putting a layer of "blue board" foam insulation down over the sewer pipe, then placing the duct bank on top of that. The 1.5"-2" thick insulation was supposed to be sufficient to keep the duct bank settling from crushing the sewer pipe, assuming the sewer pipe is bedded properly.

The other side of this challenge is protecting the duct bank from which vehicular traffic, which is now 12-18" over the top of the duct bank instead of the desired 30-36". I have seen the top of the duct bank increased to 4" thick and #3 or #4 rebar used to reinforce the duct bank. The rebar is run parallel to the duct bank with one bar near each edge and bars at 12" spacing in the middle as needed for the width of the duct bank. Then bars are bent into U shapes and two U's used together to encircle the ducts. These reinforcements that encircle the duct bank were placed every 6 feet. I don't recall what concrete mix was used in this case, I would guess 3000 psi.

All of this is just the reminiscences of an EE talking about structural matters, but it may be useful food for though to those who understand concrete better than I do.


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